1. Field of the Invention
This invention relates generally to systems for drilling boreholes for the production of hydrocarbons and more particularly to a drilling system having an acoustic measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) system as part of a bottomhole assembly for measuring acoustic velocities of subsurface formations during drilling of the wellbores and determining the location of formation bed boundaries around the bottomhole assembly. Specifically, this invention relates to use of adaptive filtering of acoustic signals for suppression of tool mode signals.
2. Description of the Related Art
Acoustic measurements have been used in wireline borehole logging for the past four decades. The first wireline acoustic instruments or xe2x80x9ctoolsxe2x80x9d were single transmitter and receiver devices which were sed to measure the velocity of the first arrival component of an acoustic wave pulse transmitted through the penetrated formation. This component was usually the compressional or xe2x80x9cPxe2x80x9d wave component. The velocity measurement, or more precisely the travel time of the wave component from the transmitter to the receiver, was used to compute formation porosity in formation evaluation applications. In addition, early acoustic logs were used in the conversion of seismic data, initially measured in the time domain, into the depth domain thereby yielding cross sectional displays of geological structure used in the industry as a guide to exploration and development drilling.
During the late 1960s and early 1970s, acoustic wireline devices became more complex and also yielded additional information. In the area of formation evaluation, multiple transmitters and receivers were introduced to reduce the adverse effects of the borehole upon the formation acoustic measurements. In the late 1970s, as the transmission rates of wireline telemetry systems increased, the full wave form of the received signal, rather than just the first arrival time, was measured at a plurality of receivers spaced axially along the primary axis of the logging tool. The analog signals were digitized downhole and digitized wave forms were transmitted to the surface for processing. Processing involved the extraction of the travel times of the compressional and shear components, as well as various tube wave components. In addition, the amplitudes of the various wave train components were determined. In formation evaluation, the full wave form information was used to obtain a more accurate and precise measure of formation xe2x80x9cacousticxe2x80x9d porosity. In addition, mechanical properties of the formation were determined by combining amplitudes of the various components of the measured acoustic wave form. This information was used to optimize subsequent drilling programs within the area, to aid in the design of hydraulic fracturing programs for the drilled well, and to greatly increase the accuracy and precision of the conversion of area seismic data from the time into the depth domain.
Much effort in the design of acoustic wireline logging tools was, and today still is, directed toward the minimization of acoustic energy transmitted directly through the body of the downhole instrument. The arrival of this energy component at the receiver or receivers usually occurs before the arrival of energy whose path traverses the formation and the borehole. The travel path is more direct and therefore shorter. In addition, the body of the tool is usually metallic and exhibits a faster acoustic travel time than the formation and the borehole. Since the latter arrivals contain parametric information of interest, the former is considered to be interference or xe2x80x9cnoisexe2x80x9d. This direct component is reduced and/or delayed by using a variety of techniques. The component is reduced by acoustically isolating transmitters and receivers from the tool body as much as possible. The arrival of this component is delayed, preferably until after the arrival of components from the formation and borehole, by increasing the effective travel path by cutting a series of alternating slots in the metallic tool body between the transmitter and receiver arrays. This portion of the tool body is commonly referred to as the isolation subsection or xe2x80x9cisolator subxe2x80x9d. In addition, various mathematical techniques have been used in the processing of full wave form data to remove the direct component of the received wave form.
In addition to noise generated by the direct transmission of acoustic energy through the wireline tool body, additional acoustic noise is generated as the tool is conveyed along the borehole wall. This noise is commonly referred to as xe2x80x9croad noisexe2x80x9d. The adverse effects of road noise are minimized using mechanical and mathematical techniques. The prior art teaches the use of many types of roller mechanical devices whereby the wireline tool is xe2x80x9crolledxe2x80x9d rather than xe2x80x9cdraggedxe2x80x9d along the borehole wall thereby reducing the magnitude of the road noise. In addition, since road noise is essentially incoherent, various mathematical methods are used in the processing of full wave form data to greatly reduce the effects of road noise.
The economic, technical, operational and safety advantages of measuring geophysical parameters as well as drilling management parameters, during actual drilling of the borehole were recognized in the early 1950s. Commercial MWD became available in the late 1970s and early 1980s. These measurements included directional information and a limited number of formation evaluation type services. Additional sensors and devices have been added during the intervening time period. In many respects, the sophistication of the sensors are comparable to their wireline counterparts in spite of the harsh conditions experienced in using such sensors in the drilling environment. It is feasible, at least in principle, to utilize multiple sensor combination measurement methods developed for wireline tools to obtain new and improved parametric measurements while drilling. Furthermore, it is feasible, in principle, to utilize additional sensors responding to drilling related parameters simultaneously with formation evaluation type sensors.
Wireline acoustic technology has been particularly difficult to adapt to MWD applications. In addition to road noise generated by the drilling assembly dragging against the wall of the borehole, there is an additional source of noise generated by the rotation of the drill bit and the drill string. Further, the slotted isolation sub technique used to isolate transmitters and receivers in wireline applications can not be used in MWD applications in that such slots would mechanically weaken the MWD acoustic subassembly to the failing point.
U.S. Pat. No. 5,780,784 to Robbins discloses a system for eliminating the tool mode signal from a received combined signal comprising both tool mode and formation mode components. A first receiver receives a signal that is a combination of the desired formation signal and the tool mode signal. A reference receiver is used for receiving a signal consisting primarily of the tool mode signal. A predictive filter is used to predict the tool mode component of the signal received by the first receiver on the basis of the reference signal, and this predicted signal is subtracted from the combined signal received by the first receiver.
A problem associated with adaptive predictive filtering is that it typically takes several cycles (transient time) of the tool mode signal for the parameters of the adaptive filter to attain values wherein the filtering becomes effective. The problem is exacerbated when the signal is non-stationary, as it is in acoustic logging. The non-stationarity means that the derivation of the adaptive filter is tracking a moving target. In acoustic logging, the early portion of the formation signal is of utmost importance and this transient time means that the suppression of the tool mode may be relatively ineffective for getting a good estimate of the inception of the formation signal. It is desirable to have a method of suppression of the tool mode signal that is effective in obtaining good estimates of the early portion of the formation signal. The present invention satisfies this need.
The present invention provides a method of suppressing the tool mode signal in an acoustic MWD assembly. An accelerometer mounted on the drillstring measures a signal that is dominated by the tool mode. This reference signal is used to obtain an estimate of the tool mode component of the signal received by a plurality of acoustic sensors on the acoustic MWD.
In one embodiment of the invention, an acoustic source is actuated on the logging tool generating both a tool mode that propagates through body of the tool and a formation mode that propagates through the borehole fluid into the formation, through the formation and back through the borehole to the tool. A hydrophone on the tool detects a signal that is a combination of the formation mode signal and the tool mode signal. A reference accelerometer proximate to the hydrophone measures a signal that is dominated by the tool mode. An adaptive filter is derived for attenuation of the tool mode component of the signal received by the hydrophone. A novel feature of the invention is that the derivation and application of the filter is done on time-reversed versions of the accelerometer and hydrophone signals.
In another aspect of the invention, the filter derivation is based upon deriving from the cross-spectral density of the reference and hydrophone signals, a transfer function relating the two signals, modifying the transfer function by zeroing out components where the spectral values of the transfer function lie below a threshold, and using the modified transfer function for removing the tool mode component from the hydrophone signal.
In another embodiment of the invention, a multiple band-reject filter is derived based upon frequency sub-bands in which the reference signal exceeds a predetermined threshold. The multiple band-reject filter is then applied to the hydrophone signals.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.